The electron microscope has been a mainstay in the study of gastroenteritis viruses. Two major groups of gastroenteritis viruses - the 27 nm Norwalk virus and the virus and the 70nm human rotaviruses - were discovered at NIH and in Australia, respectively, (as well as the hepatitis A virus at NIH, in collaboration with the Hepatitis Section) with the use of the electron microscope. It is remarkable that in this era of tissue culture virology, these agents were discovered initially without the use of an in vitro tissue culture system, since they could not be grown directly from clinical specimens in cell culture, We have applied the term, "direct virology," to this method of examining viruses from clinical specimens directly by EM. Although second and third generation tests have been developed for the detection of the Norwalk group and the rotaviruses, the electron microscope is still an indispensable tool for the study of these gastroenteritis viruses. It acts as the "supreme court" when other more practical tests or newly developed tests yield variable results. It also is important for the direct visualization of virus particles from density gradients (to establish their morphologic appearance, e.g., single or double capsid, integrity of capsid structure, and to determine presence or absence of particles or their quantitation) for visualizing the site of activity of monoclonal antibodies, and in serologic studies performed by immune electron microscopy. However, its most important and creative role is in its application for the detection of new, heretofore unknown, agents of acute infectious gastroenteritis and other diseases as well.